The present invention relates generally to voltage surge suppressors for protecting data communication lines from external power surges.
Surge suppressors are electronic devices designed to suppress potentially damaging power surges, or "spikes", across power or communication lines of electronic equipment. Two important characteristics of surge suppressor devices are their isolation voltage value (the maximum spike voltage value the device can handle) and the reaction time (the time period before the device suppresses a spike). In even a moderate power surge, a brief spike of several hundred volts may be present. Lightning surges, accidental high voltage shorts, or ground faults can create spikes as large as 1 kV to 5 kV or more are on communications lines at most locations. Due to their high degree of sensitivity and links to exposed "outside world" connections, communication equipment and communications ports (e.g., RS-232, RS-422) are especially vulnerable and require extremely fast reaction times. Depending on the sensitivity of the protected equipment, reaction times equal to or approaching zero can be necessary.
Transient surge protectors (also known as transient suppressors), are electronic elements similar to Zener diodes and are used as the main component in a number of state of the art surge suppressors. Like a bi-directional high-power zener diode, transient surge protector devices are designed to allow current to flow only when the terminal voltage across the device exceeds a certain level. A number of companies, such as Siemens and General Electric, manufacture transient surge protectors of different values. Transient surge protectors are small, inexpensive, rugged, will handle repetitive high currents (up to 6000 amps), and react fairly quickly to power spikes. However, these devices do not react quickly enough to reliably protect a communications port.
A less common electronic element used as the main component in surge protectors is an opto-isolator. Opto-isolator devices include an LED emitter and a photo-detector placed in close proximity. Incoming data signals are applied to the LED side of an opto-isolator. The LED generates a modulated light beam from the signal. On the other side, the photo-detector, usually a photo-transistor, receives the light beam and converts it back to an electrical signal. There is no electrical connection between the input and output of the opto-isolator. Opto-isolators provide protection by "galvanically" isolating (creating a signal coupling without any direct connections) ports to minimize the effects of surges and to protect ports having different ground potentials. A number of companies, again such as Siemens and General Electric, offer opto-isolators.
Although opto-isolator devices by their very nature isolate ports from power surges at all times (reaction time=0), the devices have two limitations. Heavy surges (e.g., currents&gt;100 mA), such as a lightning strike, can destroy the diodes in the opto-isolator. High voltage surges (e.g., voltages&gt;2500 V) can arc across the device. Either event can result in communications failure and damage to the equipment. Opto-isolators also require isolated power supplies.
Neither transient suppressor-based surge protectors nor opto-isolator-based surge protectors offer both the fast reaction times and large voltage handling capability necessary to reliably protect communication ports. Prior art surge protectors are either too slow to react to a voltage spike before damage occurs or not able to appropriately suppress a spike of a large magnitude. Whatever the cause, the spike still damages the `protected` port.